1. Field of the Invention
The present invention relates to an air conditioning system for a semiconductor clean room for supplying cleaned fresh air to the clean room, and more particularly, to an air conditioning system for a semiconductor clean room which removes chemical impurities by locating a chemical filter between the humidifier of an air conditioner and the clean room.
2. Background of the Related Art
A clean room is a special dust-free space where dust (floating particles) in the air is reduced to some desired cleanliness level so as to protect workpieces from being contaminated with dust. In a clean room, air-conditioning and light intensity are also regulated and noise and shock are minimized.
A fabrication line for a semiconductor device includes basic design processes such as pattern generation or reticle fabrication, a fabrication process for a wafer, an inspection process, an assembly/packaging process, a final inspection process and a quality examination process. Repetitive processes of diffusion, exposure, development, etching and diffusion are performed during fabrication of the wafer, so it is very important to control dust-like contaminants and regulate temperature and humidity so as to improve yields, and to ensure precision and reliability of semiconductor products.
The atmosphere outside of the clean room may include many particles and water vapor as well as smoke, and thus it is required to clean and filter the air before it circulates into the clean room.
FIG. 1 shows a schematic view of a conventional air conditioning system for a semiconductor clean room for removing dust as well as for regulating temperature and humidity. Outdoor air passes a first air conditioner 1 so as to be cleaned, and the resultant fresh air passes through a fresh air duct 2 located between the air conditioner 1 and recirculating air duct 11 to be supplied into the clean room 5. Before going into the clean room, the fresh air optionally passes through a ULPA filter (Ultra Low Penetration Air Filter) 4 according to the cleanliness required for the particular class of clean room.
For more efficient and economical operation, the fresh air, which was diverted before going into the clean room 5 or which passed by the clean room 5, passes through an additional second air conditioner 8 or a third air conditioner 9 for regulating air temperature and humidity once more before recirculating into the clean room. Furthermore, the fresh air may pass through a special dry air scrubber 7 for removing impurities in the fresh air.
To improve the efficiency of an air conditioning system for supplying fresh air into a clean room by means of the second air conditioner 8 and the third air conditioner 9, the system has been designed to circulate the fresh air centered about the clean room 5 via the following three circulating paths: (1) a first circulation line for cleaning the fresh air which passes through the first air conditioner 1 by way of the fresh air duct 2, the second air conditioner 8, the ULPA filter 4, the clean room 5 and the dry air scrubber 7; (2) a second circulation line for cleaning the fresh air which passes through the first air conditioner 1 by way of the fresh air duct 2, the second air conditioner 8, the ULPA filter 4, the clean room 5, the third air conditioner 9, the ULPA filter 4 for a second time, the clean room 5 for a second time, and the dry air scrubber 7; and (3) a third circulation line for cleaning the fresh air which passes through the first air conditioner 1 by way of the third air conditioner 9, the ULPA filter 4, the clean room 5 and the dry air scrubber 7.
In addition, a fresh air supply damper 3 (shown in dotted lines in FIG. 1) can be supplied beneath the floor of the clean room, and is accessible to the clean room via a grating 6 located under the lower part of the floor. The air supply damper 3 allows the fresh air in the clean room to alternatively recirculate or be discharged toward the outside.
In this system, as shown in FIG. 2, each of the first air conditioner 1, the second air conditioner 8 and the third air conditioner 9 generally has a dehumidifier 1a, a preheater 1b, a prefilter 1c, a medium filter 1d, a cooler 1e, a heater 1f, a humidifier 1g, an air blowing fan 1h and a HEPA filter (High Efficiency Particulate Air Filter) 1i in sequence from the upstream to the downstream direction of air flow, so as to let the outdoor air flow toward the clean room 5 as driven by the air blowing fan 1h. In this cleaning process, dust particles up to 0.1 .mu.m in the fresh air are removed up to 99.9999% by means of the prefilter 1c, the medium filter 1d and the HEPA filter 1i. Also, the humidity of the air is regulated by selectively operating the dehumidifier 1a and the humidifier 1g, and the temperature of the air is controlled by selectively operating the heater 1f and the cooler 1e, so that controlled fresh air is supplied into the clean room 5.
However, this conventional air conditioning system for a semiconductor clean room is not effective for the removal of chemical molecular impurities having an outside diameter in the angstrom(.ANG.) range, such as sulfur dioxide SO.sub.2, nitrogen dioxide NO.sub.2 and phosphoric acid H.sub.3 PO.sub.4, which are chemically reactive with the surface material of a wafer and to which the fabrication process of a semiconductor is sensitive. Table 1 shows the concentrations of chemical impurities in the air which flows through the air conditioning system of FIG. 1 measured at points A, B, and C.
TABLE 1 ______________________________________ The concentration distribution of phosphoric acid at each point in the air conditioning system for a semiconductor clean room (unit; .mu.g/m.sup.3) Measuring Measuring Measuring Point A Point B Point C ______________________________________ Before Passing 0.502 0.0051 0.12 a Humidifier After Passing 0.829 0.0068 0.21 a Humidifier ______________________________________
As shown in Table 1, the concentration of phosphoric acid rapidly increased at each measuring point after the air passed a humidifier 1g, which results from the fact that phosphoric acid is used as an additive so as to prevent the formation of scale in the humidifier 1g of the first air conditioner 1, the second air conditioner 8 and the third air conditioner 9.
These chemical impurities, which tend to stick to the film surface of a wafer as hydrophilic contaminants, cause short-circuits by eating into metals, change the electrical properties of the wafer, and cause defects such as formation of water spots on the surface of the wafer and fading of a pad.
As a means for removing these chemical impurities, the pore size of a filter may be made considerably smaller so as to make physical filtering practicable. However, the reduction of the pore size makes it more difficult to fabricate the filter itself, increases the filtering pressure of the air, decreases the amount of filtered air per unit time, and increases unit production cost of the filter.
The addition of phosphoric acid to the humidifier 1g may be avoided so as to substantially reduce the amount of phosphoric acid introduced into the air. However, in order to safely eliminate phosphoric acid, a special pure steam system must be used.
In light of the foregoing, a need exists for an economical system for removing these chemical impurities, without having to use a special steam system.